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T11010 Performance of the Ion-Counting An assessment of the linearity of the ion-counting Daly detector and Hamamatsu photomultiplier using NBS U500 Zenon Palacz, IsotopX Ltd, Middlewich, Cheshire, Technical NoteT11010 UK. The Daly detector was designed by N.R Daly in the 1960’s . The design uses a conversion dynode to convert incident ions into electrons. It also separates the multiplication electronics away from the ion beam preventing secondary ion production on the multiplication dynodes. The conversion surface is a highly polished, aluminised metal surface that is biased at ~25KV. The bias voltage controls the peak shape and to some extent the transmission. However, the efficiency variation is very small above 20KV. Ions impact onto the Daly conversion electrode which releases electrons. These are repelled onto a scintillator forming photons. The photons are detected by a photomultiplier which is external to vacuum. This is radically different to an electron multiplier where the active surface of the multiplier is in the line of sight of the ion beam and the multiplication stages are all in vacuum. The photomultiplier used is a Hamamatsu tube SN R6247, which replaces the earlier Photonis XP2900 tubes. Substantial modifications to the ion counting electronics have been made to ensure the multiplication electronics and discrimination are close coupled to the tube, this has eliminated “ringing” and double counting. Introduction Ion-Counting Daly Detector Ion counting detectors are used to measure extremely The main axial ion-counting system used on all Isotopx small ion currents typically between 1e-16 and 1e-13 amps. TIMS instruments is an ion-counting Daly detector. The For applications that involve large isotope ratios such as principle of operation is described in the box above. uranium or radiogenic lead, or indeed for zircon The Daly detector offers a number of advantages over geochronology where the Pb contents of individual SEM type ion-counters. It has a large dynamic range, zircons is extremely variable, it is crucial that ion-counting providing several orders of magnitude overlap with the detectors have a linear response. Faraday collectors. It has the highest gain stability Non linearity of ion-counters is due to a combination of relative to the Faraday collectors, and the largest working factors which have been discussed at length in the peak flat. Several Daly systems have been in continuous literature (e.g. Richter et. al 2001). Potentially the largest operation for over twenty years without replacement of source of error is the calibration of the deadtime (DT). any component. The deadtime is the period after a count has been The Daly on Phoenix and Phoenix X62 TIMS represents registered before another ion can be counted and ideally the next generation of ion-counting detector. Isotopx have it should be constant irrespective of count rate. re-designed the Daly to take advantage of the latest developments in photo-multipliers and in counting Typically the DT is a few tens of nanoseconds but it must electronics. be measured accurately to minimize its affects at high count rates. For example a count rate of 1e6 cps equates Compared to earlier Daly units this new system has lower to a time between counts of 1 microsecond and a 10ns noise and a larger dynamic range. The issue of ‘ringing’ deadtime would be a 1% contribution. has been eliminated which, combined with lower noise, allows a significantly more efficient rejection of false Richter et. al. have shown that at count rates greater than counts. ~20,000 cps, Secondary Electron Multipliers (SEM’s) are non-linear even after DT correction . The non-linearity The purpose of this note is to demonstrate the improved becomes larger at higher count rates and can be as high performance of this new Daly system as 1% at 6e5 cps. www.isotopx.com Technical Note T11010 Method 1.500 The isotopic standard NBS U500 was measured by peak 1.400 234 235 236 238 jumping U, U, U and U on the ion-counting 1.300 234 238 236 238 y = 1E-09x + 1.0522 Daly detector. The isotope ratios U/ U and U/ U 1.200 R2 = 0.0591 235 238 are corrected for mass fractionation using U/ U. 1.100 The certified ratio of 235U/238U is 0.99968 hence 235U and GAIN 1.000 238U are essentially the same size. Consequently any 0.900 detector non-linearity will be cancelled out when a ratio is 0.800 calculated. This means that a fractionation correction 0.700 applied using 235U/238U will have no effect on any detector 0.600 non-linearity that might be present on other isotope 0 500000 1000000 1500000 2000000 2500000 3000000 3500000 238U cps ratio’s. The sample was loaded onto the side filament of a triple Figure 1. Stability and linearity of the Faraday/Daly gain. filament and the intensity adjusted by controlling the side filament current. Measurements were made at different Daly Linearity – 234U/238U 238 intensities of U over the course of several days. Each 234U/238U data are shown in Appendix 1 and Figure 2. analysis took a minimum of 2 hours, and this was The average of all the determinations is 0.010417+/- increased for the lower count rate measurements in order 0.11% 2 RSD. The dynamic range covered is 252cps to obtain the required precision for the minor isotopes. 234U to 3.2e6 cps 238U. The mean ratio compares to the Isotopes were also simultaneously detected on the off - reference value of 0.010425 which has an error of axis Faraday collectors, such that the Faraday/Daly gain approximately 0.2%. The mean accuracy is 0.08%, so the could be determined at different count rates. Table 1 results are within error of the certified value. A linear shows the analytical sequence. regression line through the data produces an intercept of Cycle 1 Cycle2 Cycle 3 Cycle 4 0.010419. The slope of the regression line is 1e-12, H4 Faraday 238 confirming that the Daly is completely linear with one H3 Faraday 238 deadtime correction H2 Faraday H1 Faraday 235 234U/238U daly linearity Daly 234 235 236 238 0.01046 Table 1. Sequence used NBS 0.010425+/-0.18% 0.01045 y = -1.230186E-12x + 1.041864E-02 The analysis consists of peak jumping 234, 235, 236 and 0.01044 R2 = 4.881860E-02 238 on the Daly. Integration times are 5 second, 3 U n 0.01043 238 0.01042 second, 5 second and 3 second respectively. In cycle 1, U/ 235 238 0.01041 U and U are measured on the off axis Faradays. 234 0.01040 The results are shown in Appendix 1. A deadtime of 0.01039 39.5ns was used for all the data. - 500,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 238U cps Faraday/Daly Gain Figure 2. Linearity of Daly using 234U/238U The Faraday/Daly gain was determined by measuring the 235U/238U on the Faraday collectors in the first cycle, and 235Daly/238F in the second cycle. Cycle 1 is divided by Daly Linearity – 236U/238U cycle 2, to obtain the gain. 236U/238U are also shown in Appendix 1 and Figure 3. The The data shows that relative to the Faraday collector the ratio is more extreme than the 234U/238U. The average of Daly is 94.8% efficient and this efficiency remains 238 6 238 the determinations is 0.001520±0.34% 2RSD. The lowest constant from 24,000 cps of U to >3e cps of U 236U determined is 37cps. The regression line through the (Figure 1). data shows zero slope, and so the Daly is totally linear Figure 1 also gives a good indication of how linear the with the one deadtime correction. The intercept is Daly is. If we assume that the Faraday is linear then there indistinguishable from the reference value. is no change in Daly gain from 24,000 cps of 238U to >3e6 cps of 238U. Determining the linearity at low count rates is difficult because of the noise of the Faraday collectors. www.isotopx.com Technical Note T11010 Comparison with SEM detectors. Richter et al 2001, compared a number of SEM detectors commonly used on isotope ratio MS instruments. They showed that a single deadtime correction could not be applied to SEM detectors and an empirical ‘best fit’ curve must first be determined and then applied to acquired data. This became more important for counting rates above 20,000cps as above this level the non-linearity of the SEM’s under test became significant. The results for 234U/238U presented here are compared with those of Richter et. al 2001 in Figure 4. The difference in behaviour between the Daly and an SEM is very clear and highlights the significant benefits of the Daly system over the SEM’s tested by Richer et al. 236U/238U daly linearity 0.00154 0.00153 0.00153 0.00152 U n 0.00152 238 0.00151 U/ 0.00151 NBS 0.001519+/-0.5% y = -8.419562E-14x + 1.519687E-03 236 0.00150 R2 = 1.152911E-03 0.00150 0.00149 - 500,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 238U cps Figure 3. Linearity of the Daly for 236U/238U Comparison between Daly and SEM 0.01046 0.01044 0.01042 0.0104 0.01038 SEM 0.01036 DALY 0.01034 Linear (DALY) 234U/238U 0.01032 0.0103 y = -1.27977E-12x + 1.04186E-02 2 0.01028 R = 5.50044E-02 0.01026 0 500000 1E+06 2E+06 2E+06 3E+06 3E+06 4E+06 238Ucps Figure 4.
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